1. Field of the Invention
The present invention relates to a filter amplifier (an active filter) incorporated in a pulse processor used in an electron probe microanalyzer or the like. The invention also relates to an active filter having a rectangular output.
2. Description of the Prior Art
Pulse processors used to detect radiations such as X-rays and gamma rays radiated from specimens are designed to process output signals from detectors in a given manner. The configuration of one example of these pulse processors is schematically shown in FIG. 5, where a detector 4 produces a step wave which is applied to a filter amplifier (also known as an active filter) 2 included in the pulse processor, indicated by 1. The detector 4 can take any form.
Ideally, the step wave produced from the detector 4 should have the form shown at A in FIG. 6. In practice, the step wave assumes the form shown at B in FIG. 6 because of leakage occurring in the detector 4 and for other causes. Furthermore, noises are actually superimposed on the wave shown at B of FIG. 6. These noises include three kinds of noises, i.e., thermal noise, current noise and flicker noise. Normally, the time constant of the filter amplifier 2 is set to a small value mainly to have a short measuring time. In practice, it is known that the thermal noise predominates. The thermal noise is also known as delta noise. The current noise is also termed step noise.
In this way, the step wave shown at B of FIG. 6 on which noises are superimposed is applied to the filter amplifier 2. The height H of this step wave corresponds to the detected radiation and so it is necessary to precisely know the height H of the step wave.
Normally, the height of the signal applied to the filter amplifier 2 is detected and processing is performed so that the detected height is converted into a given waveform. The output signal from the filter amplifier 2 is applied to an analog-to-digital converter 3, where the height H detected by the filter amplifier 2 is converted into a digital form. This output signal from the converter 3 is sent to a processor (not shown) located in the following stage.
Usually, the actual pulse processor is equipped with various means such as a pileup rejector and a live time-correcting circuit. If plural signals are superimposed on the input signal to the pulse processor, the pileup rejector nullifies these superimposed signals. The live time-correcting circuit detects the live time. Since these means are not essential for the present invention, they will not be described below.
Generally, a Gaussian filter or a triangular filter is used as the filter amplifier 2. The noise characteristics and the counting rate characteristics vary, depending on the used filter amplifier. For example, it is known that if the set time constant of the filter is increased, the noise characteristics, i.e., the resolution, are improved but the counting rate characteristics are deteriorated. Conversely, if the set time constant is reduced, then the resolution is deteriorated but the counting rate characteristics are enhanced.
Accordingly, there is a demand for a filter amplifier which has a time constant providing the desired resolution and which, therefore, provides a maximum counting rate achievable. In order to evaluate the filter amplifier having these two conflicting factors, i.e., resolution and counting rate characteristics, some index is needed.
Goulding of California University has proposed a novel method of evaluating the filter amplifier. In particular, the product of the dead time and squares of delta noise is used as a figure of merit. He says that the figure of merit of the Gaussian filter is 9.4 and that the figure of merit of the triangular filter (also known as an active filter with a triangular output) proposed by Goulding himself is 6. It can be seen from the definition that as the Goulding's figure of merit is reduced, more desirable results are produced.
In comparing the Gaussian filter with the triangular filter, the triangular filter is preferable to the Gaussian filter as long as the figure of merit proposed by Goulding is used. We have proposed a filter amplifier having a better figure of merit (IEEE Trans. Nucl. Sci., Vol. 36, No. 1, February 1989, pp. 396-399).
This filter amplifier employs a rectangular filter and a gated integrator. Assuming that the rectangular filter is ideal, the calculated figure of merit of this filter amplifier is 4. This is the best one of the theoretical values of various filters proposed heretofore.
When the step wave as shown at B of FIG. 6 is applied to the rectangular filter (also known as an active filter with a rectangular output), it converts the step wave into the rectangular wave shown at C of FIG. 6. However, an ideal rectangular filter cannot be accomplished. Therefore, the conventional rectangular filter is approximated by plural stages of Gaussian filters, but it has been very difficult to manage the production of rectangular filters of this construction, for the following reason.
FIG. 7 shows the structure of the prior art rectangular filter. A step wave from a detector (not shown) is first differentiated by a differentiator circuit 5. Three Gaussian filters 6, 7 and 8 are connected in series and located after the differentiator circuit 5. These filters 6, 7 and 8, have mutually different time constants. For example, these filters 6, 7 and 8 have successively longer time constants in this order. The output from the differentiator circuit 5, the output from the Gaussian filter 6, the output from the Gaussian filter 7 and the output from the Gaussian filter 8 are summed up by an adding circuit 9.
The step wave delivered from the detector can be converted into an approximate rectangular wave by the structure described above. If a gated integrator is connected after the rectangular filter to constitute a filter amplifier, it is confirmed that the figure of merit of this filter amplifier is 5.07. This is described in the above-cited paper.
However, the rectangular filter shown in FIG. 7 is required to have plural stages of Gaussian filters. Therefore, the circuit configuration is rendered very complex. In addition, every circuit element, such as resistors and capacitors forming the Gaussian filters, must have very high accuracy. Therefore, it is very difficult to manage the production. In practice, it has been confirmed that where resistors and capacitors forming the Gaussian filters have poor accuracies, waveforms are distorted.
As can be seen from the foregoing, where the rectangular filter shown in FIG. 7 is used, an improved figure of merit can be obtained. However, the rectangular wave obtained by summing up the outputs from plural Gaussian filters of different time constants and the output from the differentiator circuit is merely an approximate wave. Hence, the rectangular filter is not sufficiently satisfactory.